1. Field of the Invention
This invention relates to a gene and protein which can be used for vaccination against and/or for the detection and identification of R. typhi. More particularly, the invention relates to a specific nucleotide sequence encoding a highly specific and immunogenic portion of the gene encoding the protective S-layer protein antigen of Rickettsia prowazekii and the polypeptide products of this gene. The polypeptide sequence can be utilized in diagnostic and detection assays for murine typhus and as an immunogen useful as a component in vaccine formulations against murine typhus.
2. Description of the Prior Art
Murine (endemic or flea-borne) typhus, caused by infection with Rickettsia typhi, is a zoonosis that involves rats (Rattus rattus and R. norvegicus) as the main reservoir and the oriental rat flea (Xenopsylla cheopis) as the main vector [1,2]. The infection is primarily caused by scratching the flea bitted site and self-inoculating the R. typhi-laden feces, or directly by infected flea bite [3]. The symptoms of murine typhus include fever, headache, enlarged local lymph nodes and rashes on the trunk. These clinical manifestations are non-specific and resemble many other diseases such as viral infections, typhoid fever, leptospirosis, epidemic typhus and scrub typhus [3,10]. As a result, murine typhus is frequently misdiagnosed and its incidence is probably grossly underestimated.
Murine typhus is one of the most widely distributed arthropod-borne diseases of humans and occurs in a variety of environments from hot and humid lowlands to semi-arid highlands including Australia [6], Spain [7], Indonesia [8], and southwestern United States [9] in addition to previously reported countries including China, Thailand, Kuwait, Israel, and Vietnam [3,5]. It is often found in international port cities and costal regions where rodents are common [3-5].
The diagnosis of murine typhus relies mainly on serological methods [11]. The old serological assay, Weil-Felix test, is based on the detection of antibodies to Proteus vulgaris OX-19 that contains cross reactive epitopes of Rickettsia [12, 13]. However, determination of R. typhi infection by the Weil-Felix test requires a qualitative determination and therefore somewhat subjective. Additionally, because the Weil-Felix reaction requires specialized reagents, many facilities especially in rural areas or in developing countries often may not be capable of performing the laboratory diagnosis.
Other techniques include immuno-fluorescence assay (IFA) and complement fixation (CT) tests were adapted for the detection of antibodies specific for rickettsiae [14-16]. Current serodiagnostic assays such as the ELISA, Dip-S-Ticks (DS), indirect immunofluorescent antibody (IFA) and indirect peroxidase assays [17,18] require the propagation of rickettsiae in infected yolk sacs of embryonated chicken eggs or cell cultures to prepare the antigens used in these assays. However, only a few specialized laboratories have the ability to culture and purify rickettsiae, which requires Biosafety level three (BSL-3) containment facilities. Additionally, because the organism is required for the assay, in addition to potential biosafety hazards associated with the assay, these assay methods also suffer from refrigerated storage requirements, and the problem of reproducibility associated with frequent production of rickettsial antigens.
In addition to antibody-based assays, polymerase chain reaction (PCR) amplification of rickettsial protein antigen genes has been demonstrated as a reliable diagnostic method, and genotypes can be determined without isolation of the organism [19,20]. However, gene amplification requires sophisticated instrumentation and reagents generally not available in most medical facilities especially those far forward. Based on these considerations, production of recombinant antigens of R. typhi is a logic direction for the development of serological assays and vaccine candidates for murine typhus.
R. typhi has a monomolecular layer of protein arranged in a periodic tetragonal array on its surface [21]. This crystalline layer, representing 10 to 15% of the total protein mass of the rickettsia, was identified as the immunodominant species-specific surface protein antigen OmpB. It has been isolated, purified, and biochemically characterized [22-25]. The earliest and dominant immunological responses in mice, guinea pigs, rabbits, and humans, following infection with R. typhi, are directed against Omp B [17, 4, 25]. We have shown that purified native typhus OmpB induces strong humoral and cell mediated immune responses. Protective immunity was elicited by typhus OmpB in guinea pig and mouse protection models [26-29].
Based on these observations, therefore, OmpB is a particularly advantageous target for developing diagnostic reagents. R. prowazekii, the etiologic agent of epidemic typhus, also belongs to the typhus group of rickettsiae and its OmpB exhibits similar antigenic and chemical structures to those of R. typhi. Therefore, cross-reactivity of antibody to OmpB between these two species is inevitable. Cross absorption of test serum is needed to distinguish between them these to species [10].
The whole ORF of OmpB codes for a polypeptide of 1642 amino acids. The native matured protein does not contain the leader peptide at the N-terminus and the β-sheet peptide at the C-terminus. The expression of the intact OmpB protein (135 kDa) has been attempted. However, the full-length product was shown to be toxic to Escherichia coli and rapidly degraded. Moreover, due to its large size and high content of β-sheet structure, refolding of the full-length gene product was not successful.